Which amino acid is positively-charged at physiological pH?

Lysine is the amino acid that is positively charged at physiological pH (around 7.4). This occurs because lysine has a side chain that contains an amine group, which can accept a proton (H+) and become positively charged under physiological conditions. The pKa of the side chain amine group is around 10.5, meaning that at physiological pH, it will be fully protonated and carry a positive charge.

In contrast, glutamic acid and aspartic acid have carboxyl side chains that are negatively charged at physiological pH due to their ability to lose a proton. Cysteine, on the other hand, has a thiol side chain that does not carry a charge at physiological pH, as its pKa is around 8.3, meaning it remains largely uncharged under these conditions. Thus, lysine stands out as the only amino acid among the options provided that maintains a positive charge at physiological pH.